Publish the API's effective timezone and tolerate an older snapshot payload

TZ-002: with no zone stored the API reads its clock in the host's local
time, but the panel fell back to Home Assistant's zone and the admin app
to the browser's, so dates could disagree near midnight. The API now
names the zone it actually uses (api/host_zone.py: the stored zone, else
the host's own IANA zone from TZ, /etc/localtime or /etc/timezone) and
publishes it as the panel snapshot's `timezone` and the admin settings'
read-only `effective_timezone`. Both cards and the admin app format in
that one zone; Home Assistant's or the browser's zone applies only when
none is published.

TZ-003: snapshot_from_api_payload reads `timezone` with a "" default, so
an integration upgraded ahead of its API keeps refreshing against the
pre-change payload.

Task 7e173881-fb99-426f-a2f0-2678439ffc39

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

custom_components/nestquest/coordinator.py

@@ -111,8 +111,9 @@ def snapshot_from_api_payload(payload: dict[str, Any]) -> NestQuestSnapshot:
       through verbatim;
     - ``present`` and ``next_present`` pass through verbatim (the API
       resolved them through the same presence engine server-side);
-    - ``timezone`` (the stored application timezone, ``""`` when unset)
-      passes through verbatim for the cards' date and clock;
+    - ``timezone`` (the API's effective application timezone) passes
+      through verbatim for the cards' date and clock; an API older than
+      the field omits it, which reads as ``""`` (the cards' fallback);
     - the instance list is rebuilt IN ORDER, so the panel payload the
       sensors emit matches the route's order byte for byte.
 
@@ -163,7 +164,7 @@ def snapshot_from_api_payload(payload: dict[str, Any]) -> NestQuestSnapshot:
         today_iso=payload["today_iso"],
         children=tuple(children),
         cycle_day=payload["cycle_day"],
-        timezone=payload["timezone"],
+        timezone=payload.get("timezone", ""),
     )